A number of catalysts have been found for the polymerization of olefins. Some of the earliest catalysts of this type resulted from the combination of certain transition metal compounds with organometallic compounds of Groups I, II, and III of the Periodic Table. Due to the extensive amounts of early work done by certain research groups many of the catalysts of that type came to be referred to by those skilled in the area as Ziegler-Natta type catalysts.
The most commercially successful of the so-called Ziegler-Natta catalysts have heretofore generally been those employing a combination of a transition metal compound and an organoaluminum compound.
The literature shows that some work was done using Group I metal alkyls in combination with a transition metal compound. Friedlander and Oita in Ind. and Eng. Chem. 49, 1855 (1957), disclosed the polymerization of ethylene using n-butyl lithium and titanium tetrachloride. They reported that the activity of the catalyst was very dependent upon the ratio of Li to Ti and upon whether or not the two catalyst components were mixed in the presence or absence of the monomer. When the catalyst was formed in the presence of ethylene, a peak in productivity of about 850 grams of polyethylene per gram of Ti was observed when the Li to Ti gram atom ratio was about 4.5. At higher Li to Ti ratios the activity was observed to drop to a value of about 400 grams of polyethylene per gram of Ti. If the catalyst was formed in the absence of ethylene, virtually no activity was observed except at Li to Ti gram atom ratios of about 1.5, and even then the productivity was only about 190 grams of polyethylene per gram of Ti.
Frankel, Rabani, and Zilkha in J. Poly. Sci., 28, 387 (1958), reported some additional work on alkyl lithium/titanium tetrachloride polymerization catalysts. They also noted that the activity was affected by the Li/Ti ratio. They observed that peak activity of about 70 grams of polyethylene per gram of titanium occurred at Li to Ti gram atom ratio between about 2 and about 2.5 for catalyst prepared from n-butyl lithium in the absence of ethylene. Frankel et al used pressures in the range of atmospheric to about 70 psi. Higher pressures resulted in a somewhat increased polymer yield; however, the increase of the pressure range from about 40 to 50 psi to about 60 to 70 psi did not provide any significant increase in polymer yield.
The polymerization activities reported for those alkyl lithium/titanium tetrachloride catalyst systems were much lower than those of the organoaluminum/titanium tetrachloride catalysts. Perhaps for that reason little work has been done on such catalysts since that time.
It is known that even butyl lithium alone can result in some polymerization of ethylene; however, when used alone it is not considered that it would produce any significant amount of solid polymer. U.S. Pat. No. 3,429,864 discloses that the combination of alkyl lithium and a rare earth metal halide is suitable for the production of solid polyethylene; however, even here the activity of such alkyl lithium catalysts were much lower than the Zeigler-Natta type catalysts prepared from a combination of a transition metal compound and an organoaluminum compound.
An object of the present invention is to provide improved alkali metal based polymerization catalyst systems.
Another object of the present invention is to provide new and improved methods for polymerizing olefins.
Other objects, aspects, and advantages of the present invention will be apparent to those skilled in the art upon reading the following disclosure.